The present invention is broadly concerned with lifting equipment and, more particularly, with a hoist brake mechanism which is actuated by directional fluid shear to control lowering of a heavy load.
Construction and other activities involve the lifting and repositioning of heavy equipment, structural members, building materials, and the like. Hoisting apparatus typically includes a cable drum mounted on a framework, a motor to rotate the cable drum to lift the load, and a brake engaged with the drum shaft to control the lowering and stopping of the load. The hoisting apparatus may be mounted on or connected to a boom which may be swung about to a desired location for lifting or lowering of a load. During lifting, the motor is engaged with the shaft, as by a gearing and/or clutch arrangement, to rotate the shaft and lift the load. When the load reaches its maximum desired height, the brake is applied to halt rotation of the shaft, as the motor is stopped or disengaged. The brake is used to hold the load while the load is swung to a desired location. The brake is then partially released to lower the load to its new location. As the load approaches its location, the brake is tightened to slow and then stop the load, to slowly set the load down at its final location.
During a long lowering operation, the load is not allowed to simply free-fall, especially with a particularly heavy load, since the load would likely accelerate out of control. Instead, the brake is partially applied to lower the load at approximately a constant speed. Because the kinetic energy of the lowering load is converted to heat in the brake, hoisting equipment is rated on the amount of weight that can be lifted and lowered in a given amount of time, to account for the necessary dissipation of heat. The rating also factors in the horsepower of the lifting motor and the strength of the various components of the hoist equipment. The working rating of hoist mechanisms of a given design can be increased by various methods for dissipating the heat generated in lowering a load, such as by the circulation of air or fluids through the brake.
In order to increase the safety of hoist mechanisms, various methods for automatically applying the hoist brake during lowering of a load have been developed to control descent of the load. In some arrangements, the direction of rotation and sometimes the torque on the shaft are detected electronically and used to control the application of the brake. Such systems tend to be complex. In an approach disclosed in U. S. Pat. No. 3,486,588, a hoist shaft engages a helical cam through a planetary gear set which causes an actuation sleeve to compress a brake disc stack when the shaft rotates in the lowering direction of the hoist and retracts the sleeve when rotating in the lift direction to allow free rotation of the shaft. However, this is a complex arrangement involving a substantial number of components which do not directly retard rotation of the shaft.
Hoist brake arrangements are generally designed for lifting loads up to a stated upper load limit. The limit is based on the strength of the components and on the amount of braking friction that can be generated. Over time, braking friction creates wear on the brake elements, such as on stacks of rotary and fixed brake discs. If a hoist employing such a brake is typically used for lifting loads which are significantly below the upper load limit, wear often occurs on more brake elements than is necessary. However, with most hoist brake designs, there is no way to vary the number of brake elements to an optimum number for the size of the load normally lifted by the hoist.